Recently, based on the Blu-ray disc (trademark) standard that uses a bluish-purple semiconductor laser, an optical disc having a recording capacity of about 50 GB has been commercialized as a general optical disc. In the near future, it is expected that an optical disc having a large capacity of 100 GB to 1 TB equivalent to the capacity of a hard disk drive (HDD) will be developed.
However, in order to realize such an extremely high density of capacity in the optical disc, a high densification technique based on a new method different from a high densification technique based on wavelength shortening and increasing of a numerical aperture (NA) of an objective lens is necessary.
With the progress of studies related to a next-generation storage technique, a hologram recording technique that records digital information using holography has attracted attention.
The hologram recording technique refers to a technique that records information on a recording medium by superimposing signal light having information of page data two-dimensionally modulated by a space light modulator with reference light inside the recording medium and by causing modulation of a refractive index in the recording medium by a fringe pattern generated at this time.
In reproduction of the information, the reference light used in the recording is applied onto the recording medium, so that the hologram recorded on the recording medium acts like a diffraction grating to generate diffraction light. The diffraction light includes the recorded signal light and phase information, and is reproduced as the same light.
The reproduced signal light is two-dimensionally detected at a high speed using a light detector such as a CMOS or a CCD. In this way, in the hologram recording technique, it is possible to instantly record the two-dimensional information on an optical recording medium by one hologram, and to reproduce the information. Further, it is possible to overlay plural pieces of page data in a certain part of a recording medium, and it is thus possible to achieve recording and reproduction of information with a large capacity at a high speed.
As the hologram recording technique, for example, there is PTL 1. PTL 1 discloses that “FIG. 5 is a graph illustrating a time response characteristic of diffraction efficiency with respect to a waiting time from the end of recording to post treatment (time up to the post treatment) in a typical hologram recording medium using a photopolymer. As shown herein, if the time up to the post treatment is short, sufficient diffraction efficiency cannot be obtained. Thus, it is possible to determine a time necessary for a monomer to be sufficiently diffused from a diffusion speed of the monomer detected as a state of the hologram recording medium to perform sufficient refractive index modulation as an ideal time up to the post treatment”.